Continuously Cast Mg Brass

ABSTRACT

A quantity of continuously cast Mg brass is made by the step of melting a charge of Mg brass and then continuously casting a rod of the Mg brass through a casting die. The casting die has been previously treated by continuously casting a melt of copper or brass through it to clean out Mg deposits formed by an earlier continuous casting of Mg brass which formed the Mg deposits. The quantity of continuously cast Mg brass may be in the form of EDM wire. These methods may be used to purify scrap or used brass, by removing metal oxides to improve the material quality, thus reducing the need to use premium priced virgin copper and zinc.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent application Ser. No. 16/946,938 filed Jul. 13, 2020, which is a divisional of U.S. patent application Ser. No. 16/495,430 filed Sep. 19, 2019, which is a national stage filing of International Patent Application No. PCT/US19/017914 filed Feb. 14, 2019, which claims benefit of U.S. patent application 62/633,631 filed Feb. 22, 2018 and U.S. patent application 62/724,653 filed Aug. 30, 2018, each of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The inventions described herein are in the field of continuously casting alloys.

BACKGROUND ART

It has been discovered that additions of magnesium (Mg) to brass provide an alloy that gives improved performance when formed into a wire for electric discharge machining (EDM). The brass may have zinc (Zn) concentrations in the range of 5 wt % to 50 wt %. Suitable magnesium additions may be in the range of 0.02 wt % to 5 wt %. The balance of the alloy is copper (Cu) and inevitable impurities. The concentration of copper in the balance may be in the range of 45 wt % to 95 wt %. We refer to alloys with compositions in this range as “magnesium brass” or “Mg brass”.

It is difficult to make Mg brass EDM wire using conventional continuous casting systems and methods designed to produce pure brass EDM wire. The Mg tends to separate out from the alloy when it is melted. Deposits tend to form on casting dies. The wire itself tends to be more difficult to coil and draw into a fine wire suitable for EDM. EDM wires typically have a diameter in the range of 0.1 mm to 0.3 mm. Larger and smaller diameters may be suitable for different applications. Hence there is a need for an improved system and method for producing Mg brass EDM wires.

SUMMARY OF INVENTION

The summary of the invention is a guide to understanding the invention. It does not necessarily describe the most generic embodiment.

FIG. 1 is a schematic of an improved system 100 for producing Mg brass EDM wires. The system comprises:

-   -   a) a melting furnace 110 comprising:         -   i. a heated body 102;         -   ii. a cover 104;         -   iii. a source 106 of an inert gas adapted to purge said             melting furnace of air; and         -   iv. a mixer 108;     -   b) a holding furnace 130 comprising:         -   i. a body 122;         -   ii. a cover 124;         -   iii. a source 126 of an inert gas adapted to purge said             holding furnace of air; and         -   iv. a casting die 132;     -   c) an annealing furnace 150 comprising:         -   i. a heated body 142; and         -   ii. a source 144 of an inert gas adapted to purge said             annealing furnace of air; and     -   d) one or more drawing dies 170         wherein said system is adapted to make a Mg brass EDM wire by         the steps comprising:     -   e) add a bulk charge 112 of copper and zinc to said melting         furnace;     -   f) add an additive charge 114 of magnesium to said melting         furnace;     -   g) heat said bulk charge and said additive charge until they         form a melt of Mg brass;     -   h) stir 101 said melt with said mixer;     -   i) tap 131 said melting furnace to transfer said melt of Mg         brass to said holding furnace;     -   j) cast said melt of Mg brass through said casting die to form a         solid rod 141 of said Mg brass;     -   k) anneal said rod in said annealing furnace; and     -   l) draw said annealed rod through said one or more drawing dies         to form said Mg brass EDM wire 161.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic of an improved system and method for producing Mg brass EDM wire.

FIG. 2 is a schematic of a system and method for removing deposits comprising Mg from a casting die and recycling said deposits into a subsequent melt of Mg brass.

BEST MODE FOR CARRYING OUT THE INVENTION

The detailed description describes non-limiting exemplary embodiments. Any individual features may be combined with other features as required by different applications for at least the benefits described herein. As used herein, the term “about” means plus or minus 10% of a given value unless specifically indicated otherwise. As used herein, the term “substantially” means at least 90% of a desired value unless specifically indicated otherwise.

A portion of the disclosure of this patent document contains material to which a claim for copyright is made. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but reserves all other copyright rights whatsoever.

As used herein, the term “shaped” means that an item has the overall appearance of a given shape even if there are minor variations from the pure form of said given shape.

As used herein, the term “generally” when referring to a shape means that an ordinary observer will perceive that an object has said shape even if there are minor variations from said shape.

As used herein, relative orientation terms, such as “up”, “down”, “top”, “bottom”, “left”, “right”, “vertical”, “horizontal”, “distal” and “proximal” are defined with respect to an initial presentation of an object and will continue to refer to the same portion of an object even if the object is subsequently presented with an alternative orientation, unless otherwise noted.

Referring again to FIG. 1 , the bulk charge 112 may comprise a mixture of copper and zinc with 5 wt % to 50 wt % of the total charge being zinc. The total charge is the bulk charge plus the additive charge. Alternatively, the zinc may be in the range of 30 wt % to 40 wt % of the total charge. Alternatively, the zinc may be about 35 wt % of the total charge. Alternatively, the zinc may be about 39 wt % of the total charge.

The additive charge may comprise a charge of magnesium in a container of copper or brass. The charge of magnesium may be in the range of 0.02 wt % to 5 wt % of the total charge. The charge of magnesium may be in the range of 0.05 wt % to 0.5 wt % of the total charge. The charge of magnesium may be about 0.1 wt % of the total charge. The bulk charge may be added to the melting furnace first and then melted. The additive charge may be added after the bulk charge has melted. Alternatively, a portion of the bulk charge may first be melted with the additive charge, followed by later addition of the remainder of the bulk charge. For example, the copper portion of the bulk charge may be added to the melting furnace. An additive charge of magnesium may be added to the melting furnace to form a copper magnesium alloy. Then the zinc portion of the bulk charge may be added to the furnace. Another additive charge of magnesium may be added if desired.

The mixer may stir the melt after the additive charge is added to the melted bulk charge to reduce the separation of the Mg from the melt. Mixing may be done by any means such as a paddle mixer 109 illustrated in FIG. 1 . Mixing may be done alternatively or in combination with any mechanical mixer, any gas mixer (e.g. a bubbler), or any induction mixer (e.g. inductive coupling between the melt and an induction coil in proximity to or integral to the melting furnace).

The cover 104 may be placed on the melting furnace and the space below the cover of the melting furnace may be purged with an inert gas. As used herein, an “inert gas” is any gas mixture with an oxygen concentration less than that of air. For example, a mixture of nitrogen with 1 vol % oxygen produced by a membrane nitrogen generator is considered inert. An inert gas may comprise reducing gases such as hydrogen or carbon monoxide. In some embodiments, gas may be evacuated from the melting furnace (e.g., the melting furnace may be held under vacuum. In some embodiments, a carbonaceous coating material (e.g., charcoal) may be used to cover the molten melt to avoid oxygen absorption.

After the additive charge and bulk charge have been melted, the melting furnace may be tapped 131 and the melt transferred to the holding furnace 130. The holding furnace may comprise a body 122 which may be heated. The holding furnace may further comprise a cover 124 and a source 126 of an inert gas. The inert gas for the holding furnace may or may not be the same composition as the inert gas for the melting furnace. For example, the inert gas for the melting furnace may be argon and the inert gas for the holding furnace may be nitrogen.

The holding furnace may further comprise one or more vents 128 and a casting die 132. The holding furnace may further comprise a tilt mechanism 138 so that the holding furnace may be tilted as it empties to provide a constant head pressure at the casting die. As the holding furnace empties, a new bulk and additive charge may be added to the melting furnace and melted to produce a new melt. Before the holding furnace is emptied, the new melt may be transferred to said holding furnace to keep the casting process running continuously. The tilt mechanism may adjust so that the head pressure at the casting die is constant.

After the rod 141 is cast, it may be fed directly into an in-line annealing furnace. The annealing furnace may be purged with an inert gas. The inert gas for the annealing furnace may be different than the inert gasses for either the melting furnace or holding furnace. The inert gas for the annealing furnace, for example, may comprise nitrogen and about 1 vol % hydrogen.

Alternatively, the rod may be coiled after it is cast. The coiled rod may then be fed into a batch annealing furnace, such as a bell furnace. Coiling the rod allows it to be stored so that it can be drawn down to a wire at a later time.

After the rod is annealed, it may be passed through one or more drawing dies 170 to form a quantity of Mg brass EDM wire 161. The system may comprise a plurality of drawing dies with progressively smaller diameters. The step of drawing said annealed rod may comprise the steps of re-drawing 163 said rod through each of said plurality of drawing dies. The step of drawing said annealed rod may further comprise the step of re-annealing 165 said rod after it has been drawn through one or more of said plurality of drawing dies. For example, the rod may be re-annealed after being drawn through three drawing dies. The re-annealing may be done in a different annealing furnace (not shown) than the annealing furnace 150 that was initially used to anneal the cast rod 141. The different annealing furnace may be a batch furnace (e.g. a bell furnace) or an inline furnace (e.g. a double open-ended furnace).

Once the Mg brass wire has reached its desired final diameter, it may be coiled and shipped. The continuous casting process described herein enables production of large batches of EDM wire. In exemplary embodiments, the Mg brass EDM wire has been continuously cast in batch quantities of between about 1000 lb to about 20,000 lb size depending on the size of the continuous casting machine.

Flushing Deposits from Holding Furnace

It has been found by experiment that when Mg brass is cast from a holding furnace, deposits 134, 136 may be formed around the vents and casting die respectively. The deposits may comprise magnesium, copper, zinc, or oxides of copper, zinc, or magnesium. As described herein, such deposits may be recycled into a subsequent melt.

FIG. 2 is a schematic of a system and method 200 for removing the Mg deposits and recycling them for a future Mg brass melt. It has been surprisingly found that the deposits can be removed by the steps of:

-   -   a) after a melt of Mg brass has been cast into a rod, add a         second bulk charge 212 of flushing metal to the melting furnace         110, said flushing metal being operable to dissolve the deposits         that may have formed on the casting die and/or vent;     -   b) heat said second bulk charge to form a melt of flushing         metal;     -   c) transfer 231 said melt of flushing metal to said holding         furnace 130; and     -   d) cast a rod 241 of flushing metal from said flushing melt         through said casting die 132 such that said deposits that may         have formed on said casting die and/or said vent are removed         234, 236 and dissolved in said flushing melt.         Said rod of flushing metal may be formed into a coil 204.

The flushing metal may be brass substantially comprising copper and zinc at about the desired concentrations in said Mg brass wire. The coil may then be returned 202 to said melting furnace and melted for a second melt of Mg brass. The composition of said flushing metal may be measured and additional Mg added to the melt to achieve a desired concentration of Mg. The desired concentration of Mg in the Mg brass wire may be in the range of 0.02 wt % to 5 wt %. In embodiments, the desired concentration may be in the range of 0.02 wt % to 0.09 wt %. The second melt of Mg brass may then be transferred to the holding furnace and cast into a second rod of Mg brass. The second rod of Mg brass may then be drawn through one or more drawing dies to form a second quantity of Mg brass EDM wire.

In an alternative embodiment, pure copper is used as the flushing metal. When the flushing rod is recycled to the melting furnace, both zinc and Mg may be added to make a second melt of Mg brass.

In another alternative embodiment, the flushing melt can comprise any metal that will dissolve Mg deposits.

Casting Die

The casting die may be made from graphite or any other suitable material. It has been found by experiment that a graphite die suitable for casting a brass rod may wear out quickly when used to cast Mg brass. It has been surprisingly found that when the graphite die is coated, that the die life is substantially increased. Suitable coatings include phenolic resin and phosphorus.

Coated Wires

Mg brass EDM wires may be subsequently coated. Suitable coatings are copper, zinc, and alloys thereof. If the Mg brass EDM wires are coated with Zn, with a ratio of Cu/Zn alloy, or with Mg, they may be subsequently annealed to form any or all combinations of beta, gamma, or epsilon brass coatings, and/or to create a magnesium oxide coating. Wires may subsequently be recoated with an overlayer of Zn or Cu/Zn alloy, reannealed to form additional combinations of copper and zinc coatings to form beta, gamma, or epsilon phase or magnesium oxide coatings in any physical form, such as overlapping, covering the previous coating or coatings as another overlayer. Both coated and uncoated wires are suitable for use in EDM machines with feedback control on the cutting speed that increases the speed until wire breakage. The EDM machine then sets the cutting speed to a slightly lower value. The wires are also suitable for use in EDM machines with auto-threading. It has been found by experiment that Mg brass wires auto-thread more reliably than conventional brass wires.

EXAMPLE 1

A charge of brass was melted in a melting furnace. The copper content was about 64.5 wt %. This was about the desired copper concentration of 65 wt %. The balance of the melt was zinc and inevitable impurities. Hence the zinc content was about 35.5 wt %. This was about the desired zinc concentration of 35 wt %. Mg was added to the heat to bring the Mg content to about 0.1 wt %. This was about the desired Mg concentration of 0.1 wt %. This made a first melt of Mg brass. The first melt was transferred to a holding furnace and cast into a first rod of Mg brass. The first rod of Mg brass was annealed and drawn down to make a first quantity of Mg brass EDM wire with a diameter of about 0.25 mm.

After the first melt of Mg brass was cast, deposits were observed on the holding furnace vents and casting die. A charge of flushing metal was added to the melting furnace and melted to form a melt of flushing metal. The flushing metal had about the same copper and zinc content as the first melt of Mg brass. The flushing melt was transferred to the holding furnace and a flushing rod was cast. The deposits on both the vent and the casting die were dissolved in the flushing melt.

A user placed the first quantity of Mg brass EDM wire in an EDM cutting machine with auto-treading. Relative to regular brass wire, the Mg brass EDM wire cut 20% faster, had fewer breaks and had consistent and reliable auto-threading. While not wanting to be held to the explanation, the better auto-threading may be related to having the zinc concentration at a level of about 35 wt %. This is close to the upper limit for having a pure alpha phase brass in an Mg free brass alloy. When Mg is added, this may cause property changes that make the wire stiffer and provide more consistent auto-treading. The addition of magnesium has made a finer, more uniform, linear grain structure in the brass reducing its ductility and increasing the wire strength. By increasing the wire strength there will be a reduced chance of it breaking when under stress.

It was also observed that the metal part that was cut in the EDM cutting machine had a smoother finish than when the same metal was cut with brass EDM wire with no added magnesium. It was also observed that fewer deposits were formed within the water bath of the EDM machine relative to regular brass EDM wire.

It was also observed that there was a 20-30% increase in speed when doing subsequent skim passes, improving accuracy and surface finish using the same generator settings. It was also observed that a thinner, recast layer was present as compared to an EDM wire without Mg doping. It was also observed that the electrical arch was stronger (in other words, had more energy or was hotter) compared to comparable non-doped EDM wire using the same generator setting. It was also observed that the used wire had less physical damage (e.g., smaller sized pits) after the EDM process which may indicate that this wire appears to be running cooler since there was less heat damage to the wire during erosion. The reduced pitting on the wire indicates that more energy is being transferred to the cut than with a comparable non-doped wire.

EXAMPLE 2

Continuing with Example 1, after the flushing rod was cast, the flushing rod was transferred back to the melting furnace and melted. The Mg content was measured and enough Mg was added to bring the Mg content to about the desired concentration of 0.1 wt % to make a second melt of Mg brass. The second melt was then transferred to the holding furnace and cast into a second rod of Mg brass. The rod was then annealed and drawn through one or more drawing dies to form a second quantity of Mg brass EDM wire. The diameter of the Mg brass EDM wire was about 0.25 mm. This was in the desired range for this example of between 0.1 mm to 0.4 mm.

A user placed the second quantity of Mg brass EDM wire in an EDM cutting machine with auto-threading. Relative to regular brass wire, the second quantity of Mg brass EDM wire cut 20% faster, had fewer breaks and had consistent and reliable auto-threading. It was also observed that the article that was cut in the EDM cutting machine had a smoother finish than when the same article was cut with brass EDM wire with no added magnesium. It was also observed that fewer deposits were formed within the water bath of the EDM machine relative to regular brass EDM wire.

EXAMPLE 3

A charge of brass was melted in a melting furnace. The zinc content was about 38 wt %. The balance of the melt was copper and inevitable impurities. Hence the copper content was about 62 wt %. Mg was added to the melt to bring the Mg content to about 0.08 wt %. This made a first melt of Mg brass. The first melt was transferred to a holding furnace and cast into a first rod of Mg brass. The first rod of Mg brass was annealed and drawn down to make a first quantity of Mg brass EDM wire with a diameter of about 0.25 mm.

A user placed the first quantity of Mg brass EDM wire in an EDM cutting machine. The Mg brass EDM wire had a faster cutting speed than both a Mg brass wire having 0.1 wt % Mg content and a comparable brass wire that was not Mg-doped.

Embodiments of Mg brass EDM wire described herein may have an electrical conductivity of at least about 27.6% International Annealed Copper Standard (IACS). In other embodiments, the Mg brass EDM wire may have an electrical conductivity in the range of 27% IACS to 33% IACS.

In embodiments, magnesium is used as a deoxidation and desulphurizing agent to help remove impurities, such as oxygen, which would decrease the electrical conductivity of the EDM wire. For example, zinc oxide and/or copper oxide may react with magnesium to form magnesium oxide. Magnesium oxide will typically float to the top of the melt as dross and/or be deposited on the casting die, and is therefore readily excluded from the wire. Embodiments of Mg brass EDM wire described herein may have extremely low quantities of dissolved oxygen and/or metal oxides. By removing metal oxides (inevitable impurities) in the melt the electrical conductivity will increase, with the result that under use the wire will heat up less, stretch less, and have a reduced chance of breaking. The addition of magnesium to a brass melt is therefore extremely helpful in purifying the brass alloy.

The oxygen content of a melt prior to deoxidizing is variable and depends on the quality of the raw materials used (e.g., virgin material or scrap material), the method of manufacture, and the surrounding environment. It may be desirable to reduce the oxygen content in brass to below 0.04 wt %. In some embodiments, it may be desirable to produce a Mg brass wire with an oxygen content of less than 0.025 wt %. In one example embodiment, a brass melt had an oxygen content before deoxidizing of about 0.035 wt %. After deoxidizing, using methods described herein, the oxygen content was about 0.022 wt %. Oxygen remaining in the melt (e.g., as CuO, ZnO, MgO, or O) may be considered an inevitable impurity.

In EDM applications the wire is generally pushed to just short of the breaking point under high power (e.g., 20 amp) and high frequency (e.g., 100 kHz) conditions. A higher conductivity (lower resistance) wire tends to heat and stretch less under load. In embodiments, the continuously cast Mg brass wire has a tensile strength of at least about 900 MPa. In other embodiments, the Mg brass wire has a tensile strength in the range of 900 MPa to 1400 MPa. In other embodiments, the Mg brass wire has a tensile strength of at least about 350 MPa. The Mg brass wire may have a tensile strength, for example, of about 950 MPa, about 1050 MPa, about 1100 MPa, or about 1250 MPa. It may also be desirable to keep wire elongation to less than about 2%, to maintain accuracy and consistency of the wire while in use.

CONCLUSION

While the disclosure has been described with reference to one or more different exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt to a particular situation without departing from the essential scope or teachings thereof. For example, a rod of Mg brass may be cast vertically instead of horizontally. Therefore, it is intended that the disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention. 

1. A quantity of continuously cast Mg brass wire for electric discharge machining wherein: a) said Mg brass wire comprises: i) zinc at a first zinc concentration in the range of 5 wt % to 50 wt %; ii) magnesium at a first magnesium concentration in the range of 0.02 wt % to 5 wt %; and iii) balance copper and inevitable impurities; b) said Mg brass wire has a tensile strength of at least about 900 MPa; and c) said Mg brass wire has a diameter in the range of 0.1 mm to 0.4 mm.
 2. The quantity of continuously cast Mg brass wire of claim 1 wherein said first magnesium concentration is in the range of 0.02 wt % to 0.09 wt %.
 3. The quantity of continuously cast Mg brass wire of claim 1 wherein said Mg brass wire has an electrical conductivity of at least about 27.6% International Annealed Copper Standard (IACS).
 4. The quantity of continuously cast Mg brass wire of claim 3 wherein said Mg brass wire has an electrical conductivity in the range of 27% IACS to 33% IACS.
 5. The quantity of continuously cast Mg brass wire of claim 1, further comprising copper at a first copper concentration in the range of 60 wt % to 65 wt %.
 6. The quantity of continuously cast Mg brass wire of claim 1 wherein said Mg brass wire has a tensile strength in the range of 900 MPa to 1400 MPa.
 7. The quantity of continuously cast Mg brass wire of claim 1 wherein said inevitable impurities include an oxygen content of less than 0.025 wt %.
 8. The quantity of continuously cast Mg brass wire of claim 1 wherein said Mg brass wire is a continuously cast quantity of between about 1000 lb to about 20,000 lb size.
 9. The quantity of continuously cast Mg brass wire of claim 1, wherein said Mg brass wire has a coating comprising copper, zinc or alloys thereof.
 10. The quantity of continuously cast Mg brass wire of claim 1, wherein said first zinc concentration is in the range of 30 wt % to 40 wt %.
 11. A quantity of continuously cast Mg brass wire for electric discharge machining wherein: a) said Mg brass wire comprises: i) zinc at a first zinc concentration in the range of 5 wt % to 50 wt %; ii) magnesium at a first magnesium concentration in the range of 0.02 wt % to 0.09 wt %; and iii) balance copper and inevitable impurities; and b) said Mg brass wire has a tensile strength of at least about 350 MPa; c) said Mg brass wire has a diameter in the range of 0.1 mm to 0.4 mm.
 12. The quantity of continuously cast Mg brass wire of claim 11 wherein said Mg brass wire has an electrical conductivity of at least about 27.6% International Annealed Copper Standard (IACS).
 13. The quantity of continuously cast Mg brass wire of claim 12 wherein said Mg brass wire has an electrical conductivity in the range of 27 IACS to 33 IACS.
 14. The quantity of continuously cast Mg brass wire of claim 11 wherein said Mg brass wire has a tensile strength in the range of 350 MPa to 1400 MPa.
 15. The quantity of continuously cast Mg brass wire of claim 1 wherein said inevitable impurities include an oxygen content of less than 0.025 wt %.
 16. The quantity of continuously cast Mg brass wire of claim 1 wherein said Mg brass wire is a continuously cast quantity of between about 1000 lb to about 20,000 lb size.
 17. The quantity of continuously cast Mg brass wire of claim 1, wherein said Mg brass wire has a coating comprising copper, zinc or alloys thereof.
 18. The quantity of continuously cast Mg brass wire of claim 1, wherein said first zinc concentration is in the range of 30 wt % to 40 wt %. 